Project Title: The role of novel AIP1 isoform in pathological lymphangiogenesis Abstract The lymphatic system collects extravasated fluid, macromolecules, and immune cells from tissues and returns them to the blood circulation. VEGFR3 is critical for lymphangiogenesis. Extensive studies have thus far focused on the role of VEGFR3 in lymphangiogenesis during development, however the function, regulation and intracellular mediators of the VEGFR3-dependent pathways in lymphatic vessels during cardiovascular diseases remain poorly characterized. In our previous funding cycle, we identified a novel member of signal scaffolding molecule AIP1 as a potent regulator in pathological angiogenesis. Human genome-wide association study (GWAS) has identified an AIP1 gene variant conferring susceptibility to cardiovascular diseases including peripheral vascular disease and early onset of myocardial infarction. Our data show that mice with a global or EC-specific deletion of AIP1 exhibited enhanced inflammation, angiogenesis and arteriogenesis in ischemic tissues. To our surprise, AIP1-deficient mice exhibited reduced lymphangiogenesis, correlating with reduced expression and activity of VEGFR3 in AIP1-deficient lymphatic tissues and lymphatic endothelial cells (LECs). We have identified a novel isoform of AIP1L which is specifically expressed in LECs. AIP1L is specifically localized on cytoplasmic membrane in LECs where it strongly activates VEGFR3 signaling. We propose the following aims to define the role of the novel isoform AIP1L in pathological lymphangiogenesis: 1. Elucidate the mechanisms by which AIP1L promotes lymphangiogenic signaling. We will determine if AIP1L facilitates endocytosis and recycling to cytoplasm membrane. 2. Define the mechanism for AIP1L gene regulation in lymphatic vessels. We will determine how the RIF1/H3K9 methylation complex and LEC transcriptional factors epigenetically regulate AIP1L transcription. 3. Determine the function of AIP1L in pathological lymphangiogenesis. We will use newly created mice with AIP1L deletion or transgene to determine if the role of AIP1 isoform in pathological lymphangiogenesis and tissue repair.